Systems and methods for removing sulfur dioxide from a gas stream

ABSTRACT

Methods for removing sulfur dioxide from a gas stream are disclosed. A method may include passing a gas stream comprising SO 2  through a gas scrubbing apparatus. A scrubbing liquor comprising hydroxide ions and at least one oxidation catalyst may be flowed into the gas scrubbing apparatus, thereby contacting the gas stream with the scrubbing liquor. In response to the contacting, at least 90 wt. % of the sulfur dioxide may be removed from the gas stream. Concomitant to the contacting, at least some of the sulfur dioxide may react with at least some of the hydroxide ions, thereby forming sulfite ions in the scrubbing liquor. Some of the sulfite ions may be oxidized, via the oxidation catalyst, thereby forming sulfate ions in the scrubbing liquor. A used scrubbing liquor may be discharged from the scrubbing apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 14/338,864, filed Jul. 23, 2014, which is incorporated byreference in its entirety herein.

BACKGROUND

Gases containing pollutants, such as sulfur dioxide, are produced inmany industrial processes. For example, industrial boilers, calciners,smelters, and bake furnaces, to name a few, may produce gas streamscontaining sulfur dioxide. The United States Environmental ProtectionAgency (“EPA”) regulates sulfur dioxide air pollution in the UnitedStates. More specifically, the EPA sets air quality standards regardingsulfur dioxide (40 CFR §§50, 53, and 58). Compliance with these (andother) EPA air quality standards may require removal of at least somesulfur dioxide from a gas stream.

SUMMARY OF THE DISCLOSURE

Reference will now be made in detail to the accompanying drawings, whichat least assist in illustrating various pertinent embodiments of the newtechnology provided for by the present disclosure. Referring now toFIGS. 1-8, systems and methods for removing sulfur dioxide from a gasstream are illustrated. Referring now to FIG. 1, one embodiment of asystem (1) for removing sulfur dioxide from a gas stream is illustrated.The illustrated system (1) includes a scrubbing apparatus (100) fortreating a sulfur dioxide-containing gas stream (110). The scrubbingapparatus (100) is adapted to contact the gas stream (110) with ascrubbing liquor (160) comprising an oxidation catalyst (164) andhydroxide ions. In response to this contacting, at least 90 wt. % of thesulfur dioxide of the gas stream (110) may be removed, thereby creatinga treated gas stream (120). Concomitant to the contacting at least someof the sulfur dioxide may react with the hydroxide ions to form sulfiteions. At least some of these sulfite ions may then be oxidized, via theoxidation catalyst (164), to sulfate ions, thereby creating a usedscrubbing liquor (130). As described below, the oxidation catalyst (164)may facilitate the reaction of sulfite ions to sulfate ions, thus a highratio of sulfate ions to sulfite ions may be formed in the usedscrubbing liquor (130). The used scrubbing liquor (130) may bedischarged from the scrubbing apparatus (100) to a recycle vessel (200).The recycle vessel (200) may recirculate a portion of the used scrubbingliquor (130) back to the scrubbing apparatus (100) via a recycledscrubbing liquor (220). The recycle vessel may also discharge a recycleeffluent (230) comprising at least some of the used scrubbing liquor(130) to a reaction vessel (300).

Still referring to FIG. 1, inside the reaction vessel (300), the recycleeffluent (230) comprising at least some of the used scrubbing liquor(130) may be contacted with lime (310). As a result of this contact, atleast some of the sulfite ions and some of the sulfate ions may reactwith the lime (310) to form precipitated solids and regeneratedhydroxide ions. The precipitated solids may include solid gypsumparticles and non-gypsum solids. As described below, the precipitatedsolids may comprise at least 85% gypsum due, at least in part, to thepresence of the oxidation catalyst (164) in the scrubbing liquor (160).A reaction slurry (320) including at least some of the precipitatedsolids and regenerated hydroxide ions may be discharged from thereaction vessel (300) to a thickener (400).

Inside the thickener (400), at least some of the precipitated solids maybe settled from the reaction slurry (320), thereby forming a liquid-richlayer above a solids-rich layer inside the thickener (400). Theliquid-rich layer may include a majority of the regenerated hydroxideions. The solids-rich layer may include a majority of the precipitatedsolids. At least some of the liquid-rich layer may be decanted, via athickener effluent (420), from the thickener (400) to a scrubber feedvessel (600). A seed crystal slurry (440) comprising at least some ofthe solids-rich layer may be discharged from the thickener (400) to thereaction vessel (300). A solids recycle stream (450) comprising at leastsome of the solids-rich layer may be discharged from the thickener (400)to the scrubber feed vessel (600). A filter feed slurry (430) comprisingat least some of the solids-rich layer may be discharged from thethickener (400) to a filter (500).

Still referring to FIG. 1, the filter (500) may separate the filter feedslurry (430) into a filter effluent (530) and a wetcake material (520).The wetcake material (520) may comprise a solids portion and a liquidportion. The solids portion may include at least some of the solidgypsum particles formed in the reaction vessel (300). As describedbelow, the solids portion may comprise at least 85% gypsum due at leastin part to the high ratio of sulfate ions to sulfite ions achieved viathe oxidation catalyst (164). The filter effluent (530) may include atleast some of the regenerated hydroxide ions formed in the reactionvessel (300). The filter effluent (530) may be discharged from thefilter (500) to the scrubber feed vessel (600).

The scrubber feed vessel (600) may mix the thickener effluent (420) andthe filter effluent (530) together with the solids recycle stream (450)to form a regenerated scrubbing liquor (620). The regenerated scrubbingliquor (620) may include at least some of the precipitated solids of thesolids recycle stream (450). The solids recycle stream (450) may then bemixed with the recycled scrubbing liquor (220) to form the scrubbingliquor (160). Thus, at least some of the precipitated solids may beflowed into the scrubber (100).

Referring now to FIG. 2, one embodiment of a scrubbing apparatus (100)is shown. As used herein, a “gas scrubbing apparatus” and the like meansan apparatus for removing at least some SO₂ from a gas stream. In theillustrated embodiment, the scrubbing apparatus comprises a housing(102) having a gas inlet (190) for receiving the gas stream (110)containing sulfur dioxide (112) and oxygen (114). The scrubbingapparatus (100) also includes a gas outlet (194) for discharging atreated gas stream (120). The housing (102) may include a liquid inlet(142) interconnected to a scrubbing liquor manifold (180). The scrubbingliquor manifold (180) may comprise one or more scrubbing liquor nozzles(182) adapted to spray liquid droplets of the scrubbing liquor (160)into the housing (102). The housing (102) may also comprise one or moredemisters (148) configured to capture the liquid droplets and channelthem into a liquid outlet (144). The housing (102) may further include acooling spray manifold (170) comprising cooling spray nozzles (172) forspraying a cooling water mist into the housing (102).

As described above, the gas stream (110) contains sulfur dioxide (112)when it enters the scrubbing apparatus (100) via the gas inlet (190). Inone embodiment, the SO₂ concentration in the gas stream is from 1 to 400ppm (parts per million) when it enters the gas inlet. In anotherembodiment, the SO₂ concentration in the gas stream is from 1 to 350 ppmwhen it enters the gas inlet. In still another embodiment, the SO₂concentration in the gas stream is from 1 to 300 ppm when it enters thegas inlet. In another embodiment, the SO₂ concentration in the gasstream is from 1 to 250 ppm when it enters the gas inlet. In stillanother embodiment, the SO₂ concentration in the gas stream is from 1 to200 ppm when it enters the gas inlet.

Still referring to FIG. 2, the scrubbing liquor (160) enters thescrubbing apparatus (100) via liquid inlet (142). As used herein, a“scrubbing liquor” and the like means any liquid adapted to remove SO₂,via a gas scrubbing apparatus, from a waste gas stream via interactiontherewith. In one embodiment, the scrubbing liquor is an aqueoussodium-based scrubbing liquor adapted to be regenerated via dilute dualmode alkali methodology. As used herein, “dilute dual mode alkalimethodology” and the like means the method of removing SO₂ from a gasstream wherein the SO₂ is absorbed by an alkaline scrubbing liquor,followed by regeneration of the scrubbing liquor via reaction with lime,wherein the active sodium (Na) concentration of the scrubbing liquor isnot greater than 0.15M. As used herein, “active Na concentration” andthe like means 2×[SO₃ ⁻² ions]+[HSO₃ ⁻¹ ions]. When it enters the liquidinlet (142), the scrubbing liquor (160) may comprise hydroxide ions(162), an oxidation catalyst (164), regenerated hydroxide ions (322),water (605) and precipitated solids (324) including solid gypsumparticles (326) and non-gypsum solids (328). As used herein, “hydroxideions” and the like means OH⁻¹ ions. As described above, the scrubbingliquor nozzles (182) may spray liquid droplets of the scrubbing liquor(160) into the housing (102). The gas stream (110) comprising sulfurdioxide (SO₂) (112) and oxygen (O₂) (114) may be passed through thehousing (102) thereby contacting at least some of the SO₂ (112) with theliquid droplets. Concomitant to the contacting, at least some of the SO₂(112) may be reacted, in the housing (102), with at least some of thehydroxide ions (162) to form sulfite ions (132) according to thefollowing reactions:

SO₂+2OH⁻¹→SO₃ ⁻²+H₂O

SO₂+SO₃ ⁻²+H₂O→2HSO₃ ⁻¹

Once formed, the sulfite ions (132) may be converted, in the housing(102), to sulfate ions (134) via the oxidation catalyst (164) accordingto the following reactions:

SO₃ ⁻²+0.5O²→SO₄ ⁻²

HSO₃ ⁻¹+0.5O²+OH⁻¹→SO₄ ⁻²+H₂O

As used herein, “sulfite ions” and the like means SO₃ ⁻² and/or HSO₃ ⁻¹ions. As used herein, “sulfate ions” and the like means SO₄ ⁻² ions. Asused herein, an “oxidation catalyst” and the like means a material thatincreases the rate of oxidation of sulfite ions (SO₃ ⁻² and/or HSO₃ ⁻¹)to sulfate ions (SO₄ ⁻²). Thus, the presence of the oxidation catalyst(164) in the scrubbing liquor (160) may facilitate a high ratio ofsulfate ions (134) to sulfite ions (132) in the used scrubbing liquor(130).

The oxidation catalyst (164) may be water soluble. For example, theoxidation catalyst (164) may include water soluble forms of one or moretransition metals, for example one or more salts of a transition metal.The (164) oxidation catalyst may be at least partially dissolved in thescrubbing liquor (160). In one embodiment, the oxidation catalystcomprises iron in the plus two oxidation state (Fe²⁺). For example, theoxidation catalyst may comprise FeSO₄.7H₂O. In another embodiment, theoxidation catalyst may comprise manganese (Mn). For example, theoxidation catalyst may comprise KMnO₄. In one approach, the oxidationcatalyst comprises both Fe and Mn. For example, in one embodiment, theoxidation catalyst comprises both Fe and Mn in a ratio of approximately1:5 (Fe:Mn). In another embodiment, the oxidation catalyst comprisesboth Fe and Mn in a ratio of approximately 2:5 (Fe:Mn). In yet anotherembodiment, the oxidation catalyst comprises both Fe and Mn in a ratioof approximately 3:5 (Fe:Mn). In one aspect, the catalyst comprises oneor more of iron, manganese, chromium, cobalt, copper, iron, manganese,nickel and vanadium. In one embodiment, the scrubbing liquor maycomprise at least 2 ppm (parts per million) of the oxidation catalyst.In another embodiment, the scrubbing liquor may comprise at least 3 ppmof the oxidation catalyst. In one embodiment, the scrubbing liquor maycomprise at least 5 ppm of the oxidation catalyst. In anotherembodiment, the scrubbing liquor may comprise at least 7 ppm of theoxidation catalyst. In one embodiment, the scrubbing liquor may compriseat least 10 ppm of the oxidation catalyst. In another embodiment, thescrubbing liquor may comprise not greater than 100 ppm of the oxidationcatalyst. In yet another embodiment, the scrubbing liquor may comprisenot greater than 80 ppm of the oxidation catalyst. In anotherembodiment, the scrubbing liquor may comprise not greater than 60 ppm ofthe oxidation catalyst. In yet another embodiment, the scrubbing liquormay comprise not greater than 50 ppm of the oxidation catalyst. Inanother embodiment, the scrubbing liquor may comprise not greater than40 ppm of the oxidation catalyst. In yet another embodiment, thescrubbing liquor may comprise not greater than 30 ppm of the oxidationcatalyst. In another embodiment, the scrubbing liquor may comprise notgreater than 20 ppm of the oxidation catalyst. In yet anotherembodiment, the scrubbing liquor may comprise not greater than 15 ppm ofthe oxidation catalyst.

As described above, the oxidation catalyst (164) increases the rate ofoxidation of sulfite ions (132) to sulfate ions (134) in the housing(102). Therefore, the oxidation catalyst (164) may facilitate theformation of a high ratio of sulfate ions (134) to sulfite ions (132) inthe used scrubbing liquor (130). In one embodiment, the sulfate ions(134) and the sulfite ions (132) are present in the used scrubbingliquor (130) in a ratio of at least 19:1 (sulfate ions:sulfite ions). Inanother embodiment, the sulfate ions (134) and the sulfite ions (132)are present in the used scrubbing liquor (130) in a ratio of at least24:1 (sulfate ions:sulfite ions). In one embodiment, the sulfate ions(134) and the sulfite ions (132) are present in the used scrubbingliquor (130) in a ratio of at least 97:3 (sulfate ions:sulfite ions). Inanother embodiment, the sulfate ions (134) and the sulfite ions (132)are present in the used scrubbing liquor (130) in a ratio of at least49:1 (sulfate ions:sulfite ions). In one embodiment, the sulfate ions(134) and the sulfite ions (132) are present in the used scrubbingliquor (130) in a ratio of at least 99:1 (sulfate ions:sulfite ions). Inanother embodiment, the sulfate ions (134) and the sulfite ions (132)are present in the used scrubbing liquor (130) in a ratio of at least200:1 (sulfate ions:sulfite ions). In one embodiment, the sulfate ions(134) and the sulfite ions (132) are present in the used scrubbingliquor (130) in a ratio of at least 500:1 (sulfate ions:sulfite ions).

As described above, the scrubbing liquor (160) sprayed into the housing102 may contain precipitated solids (324). The sprayed precipitatedsolids (324) may enhance the removal efficiency of the sulfur dioxidefrom the gas stream (110). In one approach, the scrubbing liquor (160)includes from 0.1 to 5 wt. % precipitated solids (324). In oneembodiment, the scrubbing liquor (160) includes not greater than 5.0 wt.% precipitated solids (324). In another embodiment, the scrubbing liquor(160) includes not greater than 4.5 wt. % precipitated solids (324). Inyet another embodiment, the scrubbing liquor (160) includes not greaterthan 4.0 wt. % precipitated solids (324). In another embodiment, thescrubbing liquor (160) includes not greater than 3.5 wt. % precipitatedsolids (324). In yet another embodiment, the scrubbing liquor (160)includes not greater than 3.0 wt. % precipitated solids (324). Inanother embodiment, the scrubbing liquor (160) includes at least 0.1 wt.% precipitated solids (324). In another embodiment, the scrubbing liquor(160) includes at least 0.5 wt. % precipitated solids (324). In yetanother embodiment, the scrubbing liquor (160) includes at least 1.0 wt.% precipitated solids (324). In another embodiment, the scrubbing liquor(160) includes at least 1.5 wt. % precipitated solids (324). In yetanother embodiment, the scrubbing liquor (160) includes at least 2.0 wt.% precipitated solids (324).

Still referring to FIG. 2, cooling water (150) and compressed air (152)may be directed, via the cooling spray manifold (170), to the coolingspray nozzles (172). In the illustrated embodiment, the cooling spraynozzles (172) are air atomizing spray nozzles. Thus, the cooling spraynozzles (127) may mix the compressed air (152) with the cooling water(150) to produce a fine cooling mist in the housing (120). While theillustrated embodiment employs air atomizing spray nozzles (172), inanother embodiment, the cooling spray nozzles are nozzles (e.g., mistingnozzles) which do not require compressed air. In still anotherembodiment, the scrubbing apparatus is free of a cooling spray manifoldand cooling spray nozzles.

The demister (148) may collect the droplets formed by the cooling spraynozzles (172). The demister (148) may also collect the droplets andprecipitated solids (324) sprayed by the scrubbing liquor nozzles (182).The droplets and precipitated solids (324) collected by the demister(148) may be channeled through the liquid outlet (144) and discharged asused scrubbing liquor (130). The used scrubbing liquor (130) maycomprise sulfite ions (132), sulfate ions (134), oxidation catalyst(164), water (605), and precipitated solids (324) including solid gypsumparticles (326) and non-gypsum solids (328).

Referring now to FIG. 3, one embodiment of a recycle vessel (200) isshown. As used herein, a “vessel” and the like means a container havingat least one inlet and at least one outlet and adapted to contain gases,liquids, solids and combinations thereof. Oxidation catalyst (164) maybe added to the recycle vessel (200) to replenish any oxidation catalystinadvertently removed from the system (1) via, for example, the wetcakematerial (520). The recycle vessel (300) may receive the used scrubbingliquor (130). The recycle vessel (200) may discharge the recycledscrubbing liquor (220) to a mixing zone (240). The recycled scrubbingliquor (220) comprising a portion of the used scrubbing liquor (130) maybe mixed with the regenerated scrubbing liquor (620) in the mixing zone(240), thereby forming the scrubbing liquor (160). The scrubbing liquor(160) may be discharged from the mixing zone (240) to the scrubbingapparatus. As used herein, a “mixing zone” and the like means a zone formixing at least two liquid streams together. In one embodiment, themixing zone may be in the form of a vessel. In another embodiment, themixing zone may be in the form of one or more pipe components (e.g., apipe “T”). The recycle vessel (200) may discharge the recycle effluent(230) comprising at least some of the used scrubbing liquor (130) to areaction vessel (300).

Referring now to FIG. 4, one embodiment of a reaction vessel (300) isshown. The reaction vessel (300) may receive the recycle effluent (230)including the sulfite ions (132) and the sulfate ions (134). Thereaction vessel (300) may receive lime (310) from, for example a limeslaking vessel. As used herein, “lime” and the like means a materialcomprising at least one of CaCO₃, Ca(OH)₂, and CaO, and combinationsthereof. Inside the reaction vessel (300), the recycle effluent (230)may be contacted with the lime (310). As a result of this contact, atleast some of the sulfate ions (134) may react with the lime to formsolid gypsum particles (326) and regenerated hydroxide ions (322)according to the following precipitation reaction:

SO₄ ⁻²+Ca(OH)₂+2H₂O→2OH⁻¹+CaSO₄.2H₂O(S)

Concomitantly, at least some of the sulfite ions (132) may react withthe lime (310) to form non-gypsum solids (328) according to thefollowing precipitation reaction:

SO₃ ⁻²+Ca(OH)₂+0.5H₂O→2OH⁻¹+CaSO₃.0.5H₂O(S)

Non-gypsum solids (328) may also be formed via reaction between CO₂(e.g., CO₂ dissolved in the recycle effluent) and lime (310) accordingthe following precipitation reaction:

CO₂+Ca(OH)₂→H₂O+CaCO₃(S)

Thus, precipitated solids (324) comprising solid gypsum particles (326)and non-gypsum solids (328) may be formed in the reaction vessel (300).As used herein, “precipitated solids” and the like means solid particlesformed in a liquid via one or more precipitation reactions. As usedherein, “solid particles” and the like means a piece of solid material.Solid particles may be comprised of one or more kinds of material. Asolid particle may be crystalline and/or amorphous. A solid particle maybe comprised of one more smaller solid particles. As used herein, a“solid gypsum particle” and the like means a solid particle comprised ofgypsum (i.e., CaCO₄.2H₂O). As used herein, “non-gypsum solids” and thelike means solid material which is free of gypsum. Non-gypsum solids mayinclude particles comprised of, for example, CaCO₃ and/or CaSO₃.0.5H₂O.As used herein, “regenerated hydroxide ions” and the like meanshydroxide ions that have been regenerated via reaction of sulfate ionswith lime. Thus, the reaction slurry (320) including the regeneratedhydroxide ions (322), water (605) and precipitated solids (324)comprising solid gypsum particles (326) and non-gypsum solids (328) maybe formed in the reaction vessel (300). As used herein, a “slurry” andthe like means a mixture of solid particles and a liquid, wherein thesolid particles are in suspension in the liquid. As used herein, a“reaction slurry” and the like means a slurry formed as a result of areaction, such as, for example, a precipitation reaction.

As described above, at least some of the lime may be consumed via theprecipitation reactions in the reaction vessel. In one embodiment, alime utilization efficiency (i.e., the amount of lime consumed in thereaction vessel via the above-described precipitation reactions dividedby the amount of lime added to the reaction vessel) of at least 85% isachieved in the reaction vessel. In another embodiment, a limeutilization efficiency of at least 87% is achieved in the reactionvessel. In yet another embodiment, a lime utilization efficiency of atleast 89% is achieved in the reaction vessel. In another embodiment, alime utilization efficiency of at least 91% is achieved in the reactionvessel. In yet another embodiment, a lime utilization efficiency of atleast 93% is achieved in the reaction vessel. In another embodiment, alime utilization efficiency of at least 95% is achieved in the reactionvessel. In yet another embodiment, a lime utilization efficiency of atleast 97% is achieved in the reaction vessel.

As described above, the presence of the oxidation catalyst (164) in thescrubbing liquor (160) may facilitate a high ratio of sulfate ions (134)to sulfite ions (132) in the used scrubbing liquor. Therefore, when thesulfate and sulfite ions (134, 132) are precipitated out of the usedscrubbing liquor (130) to form the precipitated solids (324), then solidgypsum particles (326) (i.e., the product of the sulfate ionprecipitation reaction) may be preferentially formed over non-gypsumparticles (328) (e.g., the product of the sulfite ion precipitationreactions). Thus the precipitated solids (324) may comprise a highconcentration of solid gypsum particles (326), due at least in part tothe presence of the oxidation catalyst (164) in the scrubbing liquor(160). In one embodiment, the precipitated solids (324) may comprise atleast 85 wt. % solid gypsum particles (326). In another embodiment, theprecipitated solids (324) may comprise at least 87 wt. % solid gypsumparticles (326). In still another embodiment, the precipitated solids(324) may comprise at least 89 wt. % solid gypsum particles (326). Inanother embodiment, the precipitated solids (324) may comprise at least91 wt. % solid gypsum particles (326). In still another embodiment, theprecipitated solids (324) may comprise at least 93 wt. % solid gypsumparticles (326). In another embodiment, the precipitated solids (324)may comprise at least 95 wt. % solid gypsum particles (326).

Referring now to FIGS. 4-5, The reaction slurry (320) including theprecipitated solids (324) may be discharged to the thickener (400). Atleast some of the precipitated solids (324) may be recirculated back tothe reaction vessel (300) from the thickener (400) via the seed crystalslurry (440). As used herein, a “seed crystal slurry” and the like meansa slurry which contains solid particles for promoting the formation ofcrystals. In this regard, a seed crystal slurry may promote theformation of solid particles during a precipitation reaction. Thus, theprecipitated solids (324) of the seed crystal slurry (404) may promotethe formation of solid gypsum particles (326) and non-gypsum solids(328) in the reaction vessel (300), in accordance with the aboveprecipitation reactions.

Referring now to FIG. 5, one embodiment of a thickener (400) is shown.As used herein, a “thickener” and the like means a vessel for separatingat least some of the solid particles of a slurry from the liquid of theslurry. In one embodiment, a thickener may use gravity to separate atleast some of the solid particles from the liquid. In anotherembodiment, a thickener may use centrifugal force to separate at leastsome of the solid particles from the liquid. As described above, thethickener (400) may receive the reaction slurry (320) from the reactionvessel (300). Inside the thickener (400), at least some of theprecipitated solids (324) may be settled from the reaction slurry (320),thereby forming a liquid-rich layer (402) above a solids-rich layer(404) inside the thickener (400). The liquid-rich layer (402) mayinclude water (605) and most of the regenerated hydroxide ions (322).The solids-rich layer (404) may include water (605) and most of theprecipitated solids (324). At least some of the liquid-rich layer (402)may be decanted, via the thickener effluent (420), from the thickener(400) to a scrubber feed vessel (600). As described above, at least someof the solids-rich layer (404) may be transferred, via the seed crystalslurry (440) to the reaction vessel (300). At least some of thesolids-rich layer (404) may be transferred, via the solids recyclestream (450) to the scrubber feed vessel (600). At least some of thesolids-rich layer (404) may be transferred, via the filter feed slurry(430) to a filter (500). The density of the seed crystal slurry (440)may be monitored via a seed crystal density meter (442).

Referring now to FIG. 6, one embodiment of a filter (500) is shown. Thefilter (500) may separate the filter feed slurry (430) into the filtereffluent (530) and the wetcake material (520). The filter effluent (530)may include water (650) and at least some of the regenerated hydroxideions (322). The filter effluent (530) may be discharged from the filter(500) to the scrubber feed vessel (600). The wetcake material (520) maycomprise a solids portion (522) and a liquid portion (524). The solidsportion (522) comprises at least some of the precipitated solids (324).As described above, the precipitated solids (324) may comprise a highconcentration of solid gypsum particles (326), due at least in part tothe presence of the oxidation catalyst (164) in the scrubbing liquor(160). In one embodiment, the solids portion (522) comprises at least 85wt. % gypsum particles. In another embodiment, the solids portion (522)comprises at least 87 wt. % gypsum particles. In still anotherembodiment, the solids portion (522) comprises at least 89 wt. % gypsumparticles. In another embodiment, the solids portion (522) comprises atleast 91 wt. % gypsum particles. In still another embodiment, the solidsportion (522) comprises at least 93 wt. % gypsum particles. In anotherembodiment, the solids portion (522) comprises at least 95 wt. % gypsumparticles.

Referring now to FIG. 7, one embodiment of a scrubber feed vessel (600)is shown. The scrubber feed. The scrubber feed vessel (600) may mix thethickener effluent (420) and the filter effluent (530) together with thesolids recycle stream (450) to form the regenerated scrubbing liquor(620). Hydroxide ions (162) may be added to the scrubber feed vessel(600) via a hydroxide ion stream (610). While in the illustratedembodiment, the hydroxide stream (610) enters the scrubber feed vessel(600), in other embodiments a hydroxide stream may enter at least oneof: the scrubbing apparatus (100), the recycle vessel (200), thereaction vessel (300), the thickener (400), the filter (500), and thescrubber feed vessel (600). In one embodiment, the hydroxide stream(610) comprises sodium hydroxide (NaOH).

Water (605) may be added to the scrubber feed vessel (600) via a make-upwater stream (630). While in the illustrated embodiment, the make-upwater stream (630) enters the scrubber feed vessel (600), in otherembodiments a make-up water stream may enter at least one of: thescrubbing apparatus (100), the recycle vessel (200), the reaction vessel(300), the thickener (400), the filter (500), and the scrubber feedvessel (600). In one embodiment, the make-up water stream (630) maycomprise water that contains not greater than 100 ppm (parts permillion) chlorides. In another embodiment, the make-up water stream(630) may comprise water that contains not greater than 90 ppmchlorides. In yet another embodiment, the make-up water stream (630) maycomprise water that contains not greater than 80 ppm chlorides. Inanother embodiment, the make-up water stream (630) may comprise waterthat contains not greater than 70 ppm chlorides. In yet anotherembodiment, the make-up water stream (630) may comprise water thatcontains not greater than 60 ppm chlorides. In another embodiment, themake-up water stream (630) may comprise water that contains not greaterthan 50 ppm chlorides. In yet another embodiment, the make-up waterstream (630) may comprise water that contains not greater than 40 ppmchlorides. In another embodiment, the make-up water stream (630) maycomprise water that contains not greater than 30 ppm chlorides. In yetanother embodiment, the make-up water stream (630) may comprise waterthat contains not greater than 20 ppm chlorides. In another embodiment,the make-up water stream (630) may comprise water that contains notgreater than 10 ppm chlorides. In one approach, the make-up water may bewater treated via reverse osmosis. The use of such low-chloride watermay reduce the build-up of residual chlorides in the system (1). Forexample, in one embodiment, the steady state chloride concentration inthe scrubbing liquor (160) is not greater than 35,000 ppm. In anotherembodiment, the steady state chloride concentration in the scrubbingliquor (160) is not greater than 30,000 ppm. In yet another embodiment,the steady state chloride concentration in the scrubbing liquor (160) isnot greater than 25,000 ppm. In another embodiment, the steady statechloride concentration in the scrubbing liquor (160) is not greater than20,000 ppm. In yet another embodiment, the steady state chlorideconcentration in the scrubbing liquor (160) is not greater than 15,000ppm. In another embodiment, the steady state chloride concentration inthe scrubbing liquor (160) is not greater than 10,000 ppm. In yetanother embodiment, the steady state chloride concentration in thescrubbing liquor (160) is not greater than 9,000 ppm. In anotherembodiment, the steady state chloride concentration in the scrubbingliquor (160) is not greater than 8,000 ppm. In yet another embodiment,the steady state chloride concentration in the scrubbing liquor (160) isnot greater than 7,000 ppm. In another embodiment, the steady statechloride concentration in the scrubbing liquor (160) is not greater than6,000 ppm. In yet another embodiment, the steady state chlorideconcentration in the scrubbing liquor (160) is not greater than 5,000ppm. In another embodiment, the steady state chloride concentration inthe scrubbing liquor (160) is not greater than 4,000 ppm. In yet anotherembodiment, the steady state chloride concentration in the scrubbingliquor (160) is not greater than 3,000 ppm. In another embodiment, thesteady state chloride concentration in the scrubbing liquor (160) is notgreater than 2,000 ppm (e.g., to produce high quality gypsum in thewetcake material). As used herein, “chlorides” and the like means NaCl,KCl, CaCl2 and combinations thereof. As used herein, the “steady statechlorides concentration means” the average concentration of chlorides inthe scrubbing liquor of a dilute mode dual alkali scrubber system afterthe system has been run continuously for a sufficient period of timesuch that the concentration of chlorides reaches an essentially constantlevel. Such low chloride levels in the scrubbing liquor may alsofacilitate low levels of chlorides in the wetcake material. For example,in one embodiment, the wetcake material contains not greater than 2.0wt. % Cl (chlorine), where the Cl is in the form of chlorides. Inanother embodiment, the wetcake material contains not greater than 1.9wt. % Cl, where the Cl is in the form of chlorides. In yet anotherembodiment, the wetcake material contains not greater than 1.8 wt. % Cl,where the Cl is in the form of chlorides. In another embodiment, thewetcake material contains not greater than 1.7 wt. % Cl, where the Cl isin the form of chlorides. In yet another embodiment, the wetcakematerial contains not greater than 1.6 wt. % Cl, where the Cl is in theform of chlorides. In another embodiment, the wetcake material containsnot greater than 1.5 wt. % Cl, where the Cl is in the form of chlorides.In yet another embodiment, the wetcake material contains not greaterthan 1.4 wt. % Cl, where the Cl is in the form of chlorides. In anotherembodiment, the wetcake material contains not greater than 1.3 wt. % Cl,where the Cl is in the form of chlorides. In yet another embodiment, thewetcake material contains not greater than 1.2 wt. % Cl, where the Cl isin the form of chlorides. In another embodiment, the wetcake materialcontains not greater than 1.1 wt. % Cl, where the Cl is in the form ofchlorides. In yet another embodiment, the wetcake material contains notgreater than 1.0 wt. % Cl, where the Cl is in the form of chlorides. Inanother embodiment, the wetcake material contains not greater than 0.9wt. % Cl, where the Cl is in the form of chlorides. In yet anotherembodiment, the wetcake material contains not greater than 0.8 wt. % Cl,where the Cl is in the form of chlorides. In another embodiment, thewetcake material contains not greater than 0.7 wt. % Cl, where the Cl isin the form of chlorides. In yet another embodiment, the wetcakematerial contains not greater than 0.6 wt. % Cl, where the Cl is in theform of chlorides. In another embodiment, the wetcake material containsnot greater than 0.5 wt. % Cl, where the Cl is in the form of chlorides.In yet another embodiment, the wetcake material contains not greaterthan 0.4 wt. % Cl, where the Cl is in the form of chlorides. In anotherembodiment, the wetcake material contains not greater than 0.3 wt. % Cl,where the Cl is in the form of chlorides. In yet another embodiment, thewetcake material contains not greater than 0.2 wt. % Cl, where the Cl isin the form of chlorides. In another embodiment, the wetcake materialcontains not greater than 0.1 wt. % Cl, where the Cl is in the form ofchlorides. In yet another embodiment, the wetcake material contains notgreater than 0.08 wt. % Cl, where the Cl is in the form of chlorides. Inanother embodiment, the wetcake material contains not greater than 0.06wt. % Cl, where the Cl is in the form of chlorides. In yet anotherembodiment, the wetcake material contains not greater than 0.04 wt. %Cl, where the Cl is in the form of chlorides. In another embodiment, thewetcake material contains not greater than 0.02 wt. % Cl, where the Clis in the form of chlorides. In yet another embodiment, the wetcakematerial contains not greater than 0.017 wt. % Cl, where the Cl is inthe form of chlorides (e.g., to produce high quality gypsum in thewetcake material).

Still referring to FIG. 7, the scrubber feed vessel (600) may include alevel transmitter (602) to monitor the liquid level in the scrubber feedvessel (600). As described below, the level transmitter (602) may beuseful in controlling the concentration of precipitated solids (324) inthe scrubbing liquor (160). Referring back to FIG. 5, the seed crystaldensity meter (442) may monitor the density of the seed crystal slurry(450). A solids recycle density meter (452) may monitor the density ofthe solids recycle stream (450). A solids recycle control element (454)to may control the flow rate of the solids recycle stream (450) to thescrubber feed vessel (600). In one embodiment, the solids recyclecontrol element is a valve (e.g., a modulating control valve, an“on/off” control valve, to name a few). In another embodiment, thesolids recycle control element is a modulating pump. The solids recyclecontrol element (454) may be employed to control the concentration ofprecipitated solids (324) in the scrubbing liquor (160) based on signalsfrom at least one of the seed crystal density meter (442), the solidsrecycle density meter (452), and the level transmitter (602). Forexample, the density of the solids recycle stream (450), as monitored bythe solids recycle density meter (452), may indicate the concentrationof precipitated solids (324) in the solids recycle stream (450). Thus,based on the liquid level in scrubber feed vessel (600), as indicated bythe level transmitter (602), the flow rate of solids recycle stream(450) may be adjusted in order to control the concentration ofprecipitated solids (324) in the scrubber feed vessel (600), andtherefore in the regenerated scrubbing liquor (620). By controlling theconcentration of precipitated solids (324) in the regenerated scrubbingliquor (620), the concentration of precipitated solids (324) in thescrubbing liquor (160) may also be controlled. In one approach, the flowrate of the solids recycle stream is adjusted such that the scrubbingliquor (160) includes from 0.1 to 5 wt. % of the precipitated solids(324). In one embodiment, the flow rate of the solids recycle stream isadjusted such that the scrubbing liquor (160) includes at least 0.2 wt.% of the precipitated solids (324). In another embodiment, the flow rateof the solids recycle stream is adjusted such that the scrubbing liquor(160) includes at least 0.3 wt. % of the precipitated solids (324). Inyet another embodiment, the flow rate of the solids recycle stream isadjusted such that the scrubbing liquor (160) includes at least 0.4 wt.% of the precipitated solids (324). In another embodiment, the flow rateof the solids recycle stream is adjusted such that the scrubbing liquor(160) includes at least 0.5 wt. % of the precipitated solids (324). Inyet another embodiment, the flow rate of the solids recycle stream isadjusted such that the scrubbing liquor (160) includes at least 0.6 wt.% of the precipitated solids (324). In another embodiment, the flow rateof the solids recycle stream is adjusted such that the scrubbing liquor(160) includes at least 0.7 wt. % of the precipitated solids (324). Inanother embodiment, the flow rate of the solids recycle stream isadjusted such that the scrubbing liquor (160) includes at least 0.8 wt.% of the precipitated solids (324). In yet another embodiment, the flowrate of the solids recycle stream is adjusted such that the scrubbingliquor (160) includes at least 0.9 wt. % of the precipitated solids(324). In another embodiment, the flow rate of the solids recycle streamis adjusted such that the scrubbing liquor (160) includes at least 1.0wt. % of the precipitated solids (324). In yet another embodiment, theflow rate of the solids recycle stream is adjusted such that thescrubbing liquor (160) includes not greater than 4 wt. % of theprecipitated solids (324). In another embodiment, the flow rate of thesolids recycle stream is adjusted such that the scrubbing liquor (160)includes not greater than 3.5 wt. % of the precipitated solids (324). Inyet another embodiment, the flow rate of the solids recycle stream isadjusted such that the scrubbing liquor (160) includes not greater than3 wt. % of the precipitated solids (324). In another embodiment, theflow rate of the solids recycle stream is adjusted such that thescrubbing liquor (160) includes not greater than 2.5 wt. % of theprecipitated solids (324). In yet another embodiment, the flow rate ofthe solids recycle stream is adjusted such that the scrubbing liquor(160) includes not greater than 2 wt. % of the precipitated solids(324).

As described above, precipitated solids (324) included in the scrubbingliquor (160) may facilitate increased SO₂ removal efficiency from thegas stream (110). The recirculation of precipitated solids (324)throughout the system (1) may also retard and/or prevent “scaling”(i.e., precipitation of solids) on the wetted surfaces of vessels andpiping thereof. For example, precipitated solids suspended in a liquidcontained within a vessel may provide sites for solid crystal formation,thereby preventing and/or retarding the formation of solids on thewetted surfaces of the vessel. In this regard the recycle vessel (200),reaction vessel (300), thickener (400), and/or scrubber feed vessel(600) may be equipped with one or more agitators and or baffles tomaintain the precipitated solids in suspension.

These and other aspects and advantages, and novel features of this newtechnology are set forth in part in the description that follows andwill become apparent to those skilled in the art upon examination of thefollowing description and figures, or may be learned by practicing oneor more embodiments of the technology provided for by the presentdisclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of one embodiment of a system for removingsulfur dioxide from a gas stream.

FIG. 2 is a schematic view of one embodiment of a scrubbing apparatus.

FIG. 3 is a schematic view of one embodiment of a recycle vessel.

FIG. 4 is a schematic view of one embodiment of a reaction vessel.

FIG. 5 is a schematic view of one embodiment of a thickener.

FIG. 6 is a schematic view of one embodiment of a filter.

FIG. 7 is a schematic view of one embodiment of a scrubber feed vessel.

FIG. 8 is chart illustrating data of Example 1.

DETAILED DESCRIPTION EXAMPLE 1 Evaluation of Oxidation Catalyst

Oxidation testing was performed in a batch reactor using air sparging.For each oxidation catalyst test run, a scrubbing liquor comprisingsulfite ions (Total Oxidizeable Sulfur (TOS)), water, and an oxidationcatalyst was introduced into the reactor. Comparison tests were run in asimilar manner using scrubbing liquor free of oxidation catalyst. Foreach test run, the oxidation rate was measured at various TOSconcentrations. Test runs were performed using iron (FeSO₄.7H₂O) andmanganese (added as KMnO₄) as the oxidation catalyst. FIG. 8 illustratesthe oxidation rates achieved by each of the oxidation catalysts. Asillustrated, the scrubbing liquors containing oxidation catalystsachieved higher oxidation rates than the scrubbing liquor free ofoxidation catalyst. Furthermore, the combination of about 1.1 ppm Mn andabout 5 ppm Fe as the oxidation catalyst achieved oxidation rates morethan twice as high as Mn or Fe used alone.

EXAMPLE 2 System for Removing Sulfur Dioxide From a Gas Stream

A system for removing sulfur dioxide from a gas stream similar to theone illustrated in FIGS. 1-7 was produced. The various streams of thesystem were flowed. Oxidation catalyst in the form of FeSO₄.7H₂O wassupplied to the recycle vessel at a rate of about 280 grams/hour. About42 kg/h of sulfur dioxide entered the scrubbing apparatus in the gasstream and about 4 kg/h of sulfur dioxide exited the scrubbing apparatusin the treated gas stream. The system removed more than about 90 wt. %of the sulfur dioxide from the gas stream. A lime utilization efficiencyof more than about 95% was achieved. A solid gypsum to solid calciumhydroxide ratio of at least about 24:1 was achieved in the wetcakematerial. Approximately 99 kg/h (kilograms per hour) of solid gypsum wasproduced.

While various embodiments of the new technology described herein havebeen described in detail, it is apparent that modifications andadaptations of those embodiments will occur to those skilled in the art.However, it is to be expressly understood that such modifications andadaptations are within the spirit and scope of the presently disclosedtechnology.

What is claimed:
 1. A system comprising: (a) a housing having: (i) awaste gas inlet for receiving an SO₂ containing gas stream; and (ii) atreated gas outlet for discharging a treated gas stream; (b) a liquidinlet disposed within the housing, the liquid inlet interconnected to ascrubbing liquor manifold, the scrubbing liquor manifold comprising aplurality of nozzles; (i) wherein the scrubbing liquor manifold isadapted to supply a scrubbing liquor to the plurality of nozzles, thescrubbing liquor comprising hydroxide ions and at least one oxidationcatalyst; (1) wherein the scrubbing liquor comprises at least 5 ppm ofthe oxidation catalyst (ii) wherein the plurality of nozzles are adaptedto spray liquid droplets of the scrubbing liquor into the housing toreact with and remove SO₂ from the SO₂ containing gas stream, therebyyielding a used scrubbing liquor containing sulfate and sulfite ions;(c) a recycle vessel; and (d) a demister located proximal the treatedgas outlet, wherein the demister is adapted to direct the used scrubbingliquor from the housing to the recycle vessel; (i) wherein the recyclevessel is adapted to replenish the used scrubbing liquor with theoxidation catalyst to generate a recycled scrubbing liquor; and (ii)wherein the scrubbing liquor manifold is adapted to supply the recycledscrubbing liquor to the plurality of nozzles to be sprayed into thehousing to react with the SO₂ containing gas stream.
 2. The system ofclaim 1, further comprising a reaction vessel interconnected to therecycle vessel, wherein the recycle vessel is adapted to discharge arecycle effluent to the reaction vessel.
 3. The system of claim 2,wherein the recycle effluent comprises at least some of the usedscrubbing liquor.
 4. The system of claim 3, wherein the reaction vesselis adapted to receive lime, wherein the lime contacts the recycleeffluent to form precipitated solids and to regenerate hydroxide ions,yielding a reaction slurry.
 5. The system of claim 4 further comprisinga thickener, a filter in fluid communication with the thickener, and ascrubber feed vessel in fluid communication with the thickener; whereinthe thickener is adapted to (a) receive the reaction slurry from thereaction vessel, (b) generate a thickener effluent and a filter feedslurry from the reaction slurry, and (c) discharge the filter feedslurry to the filter and the thickener effluent to the scrubber feedvessel; wherein the scrubber feed vessel is adapted to store thethickener effluent discharged from the thickener as a regeneratedscrubbing liquor in order to deliver the regenerated scrubbing liquorback to either (a) the recycle vessel, or (b) the scrubbing liquormanifold where the regenerated scrubbing liquor is combined with thescrubbing liquor to form the recycled scrubbing liquor.
 6. The system ofclaim 5, wherein the filter is adapted to separate the filter feedslurry into a filter effluent and a wetcake material, and is furtheradapted to discharge the filter effluent back to the thickener.
 7. Thesystem of claim 4 further comprising a thickener, a filter in fluidcommunication with the thickener, and a scrubber feed vessel in fluidcommunication with the thickener; wherein the thickener is adapted to(a) receive the reaction slurry from the reaction vessel, (b) generate athickener effluent and a filter feed slurry from the reaction slurry,and (c) discharge the filter feed slurry to the filter and the thickenereffluent to the scrubber feed vessel; wherein the scrubber feed vesselis adapted to store the thickener effluent discharged from the thickeneras a regenerated scrubbing liquor in order to deliver the regeneratedscrubbing liquor back to both (a) the recycle vessel, and (b) thescrubbing liquor manifold where the regenerated scrubbing liquor iscombined with the scrubbing liquor to form the recycled scrubbingliquor.
 8. The system of claim 7, wherein the filter is adapted toseparate the filter feed slurry into a filter effluent and a wetcakematerial, and is further adapted to discharge the filter effluent backto the thickener.
 9. The system of claim 1, wherein the sulfate ions andthe sulfite ions are present in the recycled scrubbing liquor in a ratioof at least 19:1 (sulfate ions:sulfite ions).
 10. The system of claim 1,wherein the sulfate ions and the sulfite ions are present in therecycled scrubbing liquor in a ratio of at least 99:1 (sulfateions:sulfite ions).
 11. The system of claim 1, wherein the sulfate ionsand the sulfite ions are present in the recycled scrubbing liquor in aratio of at least 500:1 (sulfate ions:sulfite ions).
 12. The system ofclaim 1, wherein the scrubbing liquor comprises at least 7 ppm of theoxidation catalyst.
 13. The system of claim 1, wherein the scrubbingliquor comprises not greater than 100 ppm of the oxidation catalyst. 14.The system of claim 1, wherein the scrubbing liquor comprises notgreater than 50 ppm of the oxidation catalyst.
 15. The system of claim1, wherein the scrubbing liquor comprises not greater than 10 ppm of theoxidation catalyst.
 16. The system of claim 1, wherein the oxidationcatalyst comprises water soluble forms of one or more salts of atransition metal.
 17. The system of claim 1, wherein the oxidationcatalyst comprises one or more of Fe, Mn, Cr, Co, Cu, Ni and V.
 18. Thesystem of claim 1, wherein the oxidation catalyst comprises Fe²⁺. 19.The system of claim 1, wherein the oxidation catalyst is FeSO₄.7H₂O. 20.The system of claim 1, wherein the oxidation catalyst comprises both Feand Mn.